Yes, he can. The Maharashtra Civil Service Rules and Standard Code have no restriction on Social Service if it is a non political organisation and if they are honorary in nature. However, he will not be allowed to use his influence in his position of government service to benifit the said NGO, for which he has to be careful.

He will need a permission to be office bear if he was CG employees. I am not clear about Maharashtra Govt conduct rules. Better to read rules first. It is not that he should not use official influence for the NGO rather he should not have any official dealing with the NGO.